Many hydrocarbon gas streams in addition to containing a substantial methane component also possess significant amounts of heavier hydrocarbons such as ethane, propane, butane, etc., as well as substantial amounts of acid gases such as COS, H.sub.2 S and, particularly, CO.sub.2. Frequently it is necessary to separate the acid gas, e.g., CO.sub.2 component, from the hydrocarbon component as well as separate the heavier hydrocarbon fractions from the methane. For example, in order for natural gas to be commercially acceptable, it must first meet stringent specifications with regard to heating value, hydrogen sulfide and carbon dioxide content.
There are currently a number of established technologies for the removal or partial separation of acid gases from hydrocarbon streams. Thus, it is known to use physical solvents which are selective toward the acid gas component and chemical solvents which will react with such components. Suitable examples of physical solvents include propylene carbonate, dimethylether and polyethylene glycol. Examples of suitable chemical solvents are aqueous solutions of potassium carbonate as well as a variety of amines, such as monoethanolamine, diethanolamine and the like.
Carbon dioxide flooding is a process which can be particularly useful in enhanced oil recovery processes. Carbon dioxide flooding can proceed through a variety of mechanisms, including:
(1) immiscible CO.sub.2 drive;
(2) miscible CO.sub.2 drive;
(3) hydrocarbon-CO.sub.2 miscible drive;
(4) solution gas drive;
(5) hydrocarbon vaporization; and
(6) multiple-contact dynamic miscible drive. In the L. M. Home et al, December 1974 publication in the Journal of Petroleum Technology, the CO.sub.2 properties believed to be important in causing oil displacement were stated to be the following:
(1) CO.sub.2 reduces oil viscosity;
(2) CO.sub.2 increases oil density;
(3) CO.sub.2 promotes swelling of oil;
(4) CO.sub.2 is highly soluble in water;
(5) CO.sub.2 in water has an acidic effect on limestone and carbonate rock and thus dissolves the rock;
(6) CO.sub.2 vaporizes and extracts portions of crude oil; and
(7) CO.sub.2 is transported chromatographically through porous rock.
Miscible flooding with CO.sub.2 has recently become increasingly popular, due both to its ability to recover valuable hydrocarbons from naturally occurring gas formations which were previously uneconomical to recover and also to the increased suitability for the recovery of petroleum deposits. Thus, the use of carbon dioxide for miscible flooding is gaining momentum; consequently, growing amounts of associated gas utilized in wells stimulated by this method must be processed, both in order to recover the extracted hydrocarbon content and also to efficiently recover and recycle the carbon dioxide for reinjection into the well. However, the effectiveness of CO.sub.2 is greatly reduced when contamined with impurities such as methane and nitrogen. Accordingly, an economical method for producing a rich CO.sub.2 stream substantially either reduced or free from these impurities has been a goal of the art.
A number of distillation techniques can be useful for the separation of CO.sub.2 -hydrocarbon streams, particularly at higher CO.sub.2 levels. One such attempt, the so-called Ryan-Holmes process, involves an initial separation of the methane component present from the CO.sub.2 and the heavier hydrocarbons. Since CO.sub.2 freezes at the temperatures usually encountered in a demethanizer, an alkane is fed in at the top of the column, thereby preventing freezing of the CO.sub.2. Subsequent distillation can separate CO.sub.2 from the hydrocarbon stream, and the various hydrocarbons themselves, but this leads to an expensive, involved process.
A combination distillation-membrane process has been set forth in U.S. Pat. No. 4,374,657 to Fluor, Inc.
In U.S. Pat. No. 4,417,449 a process for the separation of CO.sub.2 and sulfide gases from oil shale retorting, coal gasification, oxygen fireflooding, and CO.sub.2 miscible flood enhanced oil recovery of the off gases for various recycle processes, such as to a petroleum reservoir, is reported. The process separates the off gases into an essentially sulfur-free light BTU fuel gas, a heavy hydrocarbon stream, and a CO.sub.2 acid gas stream, with the CO.sub.2 stream being expanded in an auto-refrigeration step to provide the necessary process refrigeration.